A gradient photochromic glass article is a glass article exhibiting a gradient in photochromic properties thereacross. One example is a photochromic glass article having a first region exhibiting photochromic properties normal for a photochromic glass and a second region exhibiting photochromic properties which are at least substantially less developed than those of the first portion. Often the second portion is non-photochromic.
A number of methods for producing gradient photochromic glasses are known. Canadian Pat. No. 739,404, for example, describes a method wherein potentially photochromic glass is heat treated in a partitioned lehr having a high temperature zone and a low temperature zone. Alternately, the glass is partly encased in a refractory block during heating. Such heat treatments establish a temperature gradient across the glass which is effective to provide a gradient in photochromic properties thereacross. Thus, potentially photochromic glass in the high temperature zone of the lehr reaches a temperature sufficient to develop photochromic properties therein while the glass in the low temperature zone does not.
Similarly, U.S. Pat. Nos. 4,036,624 and 4,101,302 describe a furnace specifically designed to provide controlled graded heating. To achieve a photochromic gradient, the temperature profile of the furnace is designed so that when a glass body is heat treated, those regions of the glass where photochromism is desired are heated at a high enough temperature to develop their photochromic properties while those regions where an absence of photochromism is desired are kept cool.
A related method for producing gradient photochromic glass is described is German Patentschrift No. 2,125,232. According to that method, a continuous sheet of potentially photochromic glass is drawn past heating means which selectively heat only part of the width of the glass sheet. The heated width develops photochromic properties whereas the remainder of the sheet width does not.
U.S. Pat. No. 3,419,370 teaches a preparation of photochromic lenses by diffusing silver ions into the surface layer of a base glass and then exposing the articles to a specified heat treatment. A gradient in photochromic properties across the glass body is attainable by varying the time and/or temperature at different portions of the glass body exposed to an ion exchange medium. According to this patent the ion exchange bath contains, in all instances, silver ions (see Table 2 of the patent). The gradient in photochromic properties is achieved by causing or allowing different concentrations of silver ions to diffuse into the glass.
This method for producing gradient photochromic glass bodies is cumbersome because of the geometry and care necessary to avoid any variations in silver ion concentration, temperature or time of heat treatment which may produce undesirable irregularities in photochromic properties within the glass body.
One method recently developed for the production of gradient photochromic ophthalmic lens blanks comprises subjecting potentially photochromic glass lens blanks to a heat treatment while maintaining part of the lens blank in proximity to a quantity of evaporable liquid contained in or supported by a porous refractory carrier material. The heat treatment is carried out at a temperature sufficient to develop photochromic properties in the potentially photochromic glass; however, vapor generated by the evaporable liquid during heat treatment maintains the part of the lens blank in proximity thereto at temperatures below that required for full photochromic development. Thus, a gradient photochromic ophthalmic lens blank is provided.
Certain disadvantages arise in the production of gradient photochromic ophthalmic lens blanks by the above described method. One major disadvantage is that porous refractory carrier materials of the kind best suited for controlled vapor release are subject to deterioration and breakage due to the adverse physical and chemical effects of repeated thermal cycling in the presence of liquid.
A very important requirement of any glass product intended for ophthalmic use is uniformity with respect to clarity, color and photochromic response. Gradient photochromic ophthalmic lens blanks must meet a further requirement of uniformity in that the quality (relative sharpness) and position within the lens blank of the photochromic gradient must be reproducible. In the vapor process, it appears that rather minor variations in refractory carrier porosity and/or liquid absorption may produce variability in the gradient photochromic product.
U.S. Pat. No. 4,072,490 teaches a method for producing photochromic lenses involving the use of an apparatus with a heat sink at one end which induces a thermal gradient in the lens. The region of the lens in contact with the heat sink remains cooler than the rest of the lens for a sufficient period of time and the development of its photochromic properties are retarded. Hence, the finished product is a gradient photochromic lens.
This review of the patent literature indicates that the art of introducing graded photochromism in glass bodies is essentially limited to processes that rely on the maintenance of a temperature gradient across the glass body during heat treatment. All these processes have the same inherent limitations and problems, such as the inability to precisely control, much less choose at will, certain desirable temperature gradients (e.g. uniform step or steep gradients in low thermal conductivity materials). Another problem with thermal gradient processes is that precautions may be necessary to avoid the buildup of thermally induced stresses in the glass body due to the presence of temperature gradients. As can be deduced from U.S. Pat. Nos. 4,036,624 and 4,101,302, a temperature gradient on the order of 15.degree. C./cm is necessary to achieve distinguishable photochromic gradients in glass bodies.
It is the principle object of the present invention to provide a process for producing a step gradient photochromic glass body not possible with thermal gradient processes which are inherently limited to ramp gradients.
It is a further object to provide a process for producing a gradient photochromic glass body which is not subject to the other above-described disadvantages.
It is a further object to provide a process which produces a gradient glass body of improved uniformity with respect to gradient quality.
Other objects and advantages of the invention will become apparent from the following description and examples thereof.